It is known to provide compound or assembled pistons for internal-combustion engines, especially large displacement Diesel engines, which comprise a generally cylindrical piston body which is of tubular or solid construction and which is terminated at its end directed toward the firing chamber with a so-called piston "bottom" which may be attached by bolts or by welding to the piston body. The connecting or piston rod is articulated to the piston body and the piston bottom is generally composed of a forgeable refractory steel. The compound piston construction thus allows the body of the piston or at least other parts thereof to be composed of less expensive materials.
The piston body itself generally has a configuration which can be described as a somewhat domed or bulging (outwardly convex) central portion, an inwardly concave formation corresponding to the outwardly domed portion, and a cylindrical shoulder along the periphery of the bottom which abuts the piston body and forms an outer surface which is more or less continuous therewith although grooves may be provided in the outer cylindrical surfaces of this shoulder and the body for sealing purposes.
The bottom thus has generally a cup-shape, open toward the body of the piston and can be provided within this cup with a substantially circular boss, ridge or hub at which the bottom is joined to the body.
As noted, this abutment can be by a weld, e.g. friction welding, or by bolting.
In general the piston body is formed from forgeable refractory steel although a good part of the shape must be applied by material-removal (machining) techniques which are expensive and time-consuming.
When the bottom is formed from a blank which is initially cast from alloys having refractory characteristics, the bottom is especially sensitive to the formation of segregations which tend to concentrate at the center of the bottom or in the shoulder. Such segregations bring about anisotropies which reduce the value of the bottom and may even make it worthless. Failure can occur prematurely in use and the problem is exacerbated when heavy machining must be done and hence a possibly unusable product fabricated at especially high cost.
While forgeable alloys have been used, the degree of forging of the finished product has been limited heretofore and indeed considerable machining was necessary even where the blank was initially forged.
Thus, whether the piston bottoms were primarily cast and machined or cast, forged and machined, or even simply forged from stock obtained by other steel fabrication processes, invariably considerable effort was expended in imparting the finished shape of the product by expensive machining steps which could also exacerbate stresses within the workpiece.